Geophysical Data Grids for the Conterminous United States

Introduction

Gridded geophysical data sets for the conterminous United States are available
on CD-ROM from the U.S. Geological Survey (USGS). The data sets include magnetic
anomalies; Bouguer gravity anomalies; isostatic residual gravity anomalies;
uranium, thorium, and potassium gamma-ray measurements; and topography with
and without bathymetry. The magnetic anomaly and gamma-ray data sets are gridded
at a 2- kilometer grid interval; the gravity anomaly and topographic-bathymetric
data sets are gridded at a 4-kilometer grid interval. The gridded data are
provided in both ASCII and PC-binary formats. Potential-field geophysical software,
included with the data sets, can be used to display, process, and interpret
the gridded data.

Data Descriptions

Magnetic anomalies are produced by variations in the distribution of iron minerals,
usually magnetite, in the rocks of the Earth's crust. Igneous and metamorphic
crystalline rocks can be very magnetic. By comparison, sedimentary rocks are
usually nonmagnetic. Magnetic anomalies therefore provide a way of mapping
exposed and buried crystalline rocks.

Gravity anomalies are produced by density variations within the rocks of the
Earth's crust and upper mantle. Mapping of these density variations is the
primary use of gravity anomalies. Gravity measurements made on the surface
of the Earth must be corrected in various ways before they can be made into
an anomaly map.

The free-air correction adds to the measured gravity the predicted difference
in gravity between sea level and the elevation of the observation. The simple-Bouguer
correction subtracts the predicted effect of the mass between the observation
and sea level, using a uniform slab of constant density to model the mass.
The complete-Bouguer correction adds the predicted effect of constant-density
topography within 166.7 km of the measurement location. A gravity reference
field is subtracted from the corrected measurements to produce the free-air,
simple-Bouguer, or complete-Bouguer anomaly.

Figure Description

Red shades indicate areas of high gravity values produced by high average densities
in the Earth's crust and upper mantle; blue shades indicate areas of low gravity
values produced by low average densities. Illumination is from the west.

Data Descriptions (continued)

The grid of Bouguer gravity data for the conterminous United States and adjacent
marine areas was constructed from complete-Bouguer gravity anomaly values onshore
and free-air gravity anomaly values offshore. Because the Bouguer anomaly equals
the free-air anomaly at sea level, there is no discontinuity in the gridded
data at the shoreline.

Isostatic residual gravity anomaly maps are produced by subtracting long-wavelength
anomalies, produced by masses deep within the crust or mantle, from the Bouguer
anomaly map. Isostatic residual gravity anomaly maps therefore reveal more
clearly than Bouguer anomaly maps the density distributions within the upper
crust that are of interest in many geologic and tectonic studies.

Aerial gamma-ray surveys measure the gamma-ray flux produced by the radioactive
decay of the naturally occurring elements U-238, Th-232, and K-40, in the top
few (as much as 50) centimeters of rock or soil. When the gamma-ray system
is properly calibrated, the data can be expressed in terms of the estimated
concentrations of these radioactive elements. The element concentration data
are useful in geologic studies to identify (1) the lithology of rock outcrops,
(2) areas of high-grade metamorphism or hydrothermal alteration, (3) areas
of possible uranium mineralization, and (4) areas where high levels of radon
gas in the soil or high gamma-ray exposure levels may pose health risks.

Topographic and bathymetric data are used in the reduction and interpretation
of magnetic anomaly and gravity anomaly data and in the construction of geologic
and topographic maps.